Currently all of the Armor items added render incorrectly on Zombies and Armor Stands.
For Zombies it results in the sleeves not showing up, and armor stands 'breathe' as if they were alive. This isn't a must fix problem, but it is doable.

Sleeves have always been hidden. From a quick test, it's probably because the model does follow the mob's arms:

Base models were mixed up, all fixed in dev